Weed and grass trimmers have been developed which employ a rotatable hub with a short length of flexible nylon or other plastic line extending from the hub. When the hub is rotated, the tip of the line extending from the hub provides the cutting or trimming action. Grass and weed trimmers using this principle of operation have become popular for their versatility of use and because the flexible trim line is safer to use than rigid rotating steel blades.
Various types of devices have been developed for using such trim lines. Typically, rotating line trimmers or string trimmers employ a line which generally has a circular cross section. The line, in many trimmers, is wound on a storage reel in the hub of the device, and is played out of a hole in the hub in discrete amounts as the end breaks off or wears off.
The circular cross-sectional configuration of trimmer lines generally leaves a ragged edge on the ends of the blades of grass or plants cut with such trimmers. This is a result of the relatively blunt cutting edge, which strikes the grass and breaks it off. As a result, the ragged grass ends tend to turn brown shortly after cutting, resulting in a less desirable appearance than if the grass is cut or broken off cleanly along a straight line.
The U.S. Pat. No. 4,186,239 to Mize is directed to a modification of the normal circular cross section monofilament line used in weed and grass trimmers for the purpose of reducing terminal fibrillation of the filament. This is done by forming periodic irregular cross-sectional variations along the length of the line. This patent also discloses line extrusions of generally triangular or star shapes, in which the cutting edges are provided with notches or irregularities in planes perpendicular to the central axis of the cutting line. In all of the variations which are shown in this patent, however, a blunt edge contacts the plant to effect the cutting. Rotation of any of the lines disclosed in this patent subjects the line to symmetrical drag forces on both the upper and lower surfaces as the line is rotated. Consequently, as the line is rotated, there is nothing to cause any particular orientation of any part of the line to the grass and weeds being cut.
Several U.S. patents are directed to rotary line cutters, in which the cutting lines have triangular cross sections. Three such U.S. Pat. Nos. are Jacyno 4,118,865; Jones et al. 4,905,465; Jones et al. 5,048,278. The lines of all of these patents are of a relatively short finite length. They are not extended from a supply reel as they wear out. Theoretically, however, the triangular cross sections are intended to present a sharp cutting edge to the grass or other plants to be cut. The U.S. Pat. No. 4,126,991 to Gobin is similar to the Jacyno and Jones patents above, inasmuch as it employs a short fixed-length flexible cutting blade. The cutting blade of Gobin, however, is of a relatively complex configuration, with different cross sections at different points along its length. The end of the blade has a generally elliptical shape, with a sharp cutting edge. This cutting edge is intended to roll over onto its flat side as a result of the flexibility of the blade whenever the edge strikes an object which cannot be cut by the blade. Whenever the cutting edge of the blade of Gobin is broken or worn out, the entire fixed length blade must be replaced. Because of the non-uniform cross-sectional shape, the blade cannot be stored and played out from a supply reel.
A different approach to a modification of the conventional circular cross-sectional trimmer line is disclosed in the U.S. Pat. No. 4,869,055 to Mickelson. The line disclosed in this patent has a generally star-like cross-sectional configuration, in which longitudinally extending sharp edges are separated by depressed areas (concave or sharp angular configurations) between any two adjacent edges; so that the sharp edges always are at the outermost portion of the line. The intent is that, irrespective of the orientation of the line, a sharp edge will always strike the plant or weed first. The line is symmetrical in its cross section; and as it rotates, the wind forces which are applied to it are symmetrical; so that the drag on the upper and lower surfaces is essentially the same. Because the sharp edges always strike the plant first, the sharp edges quickly can be dulled in the use of this line, which rapidly diminishes the intended advantages in the use of such sharp edges.
It is desirable to provide an improved flexible trim line which overcomes the disadvantages of the prior art, which may be made as a uniform extrusion, and which may be used in a standard rotary hub weed and grass cutter carrying a spool of cutting line.